1. Field of the Invention
The present invention relates to a color sensor for measuring a luminance and a chromaticity of an LCD (liquid crystal display) device in calibrating the luminance and the chromaticity of the liquid crystal display device, a display system incorporated with the color sensor, and a display calibration method.
2. Description of the Related Art
It is necessary to calibrate luminance and chromaticity variations of a high-quality monitor to be used in e.g. a printing field or a medical field. In calibrating luminance and chromaticity variations of a CRT monitor, conventionally, a compact color sensor e.g. a CRT calibrator produced by Konica Minolta may be used. The CRT calibrator is constructed in such a manner that: light from a CRT monitor is received by respective corresponding silicon photodiodes of the color sensor through XYZ color filters; the light receiving signals are stored as digital signals i.e. count values in a built-in microcomputer via a current-voltage converting circuit and an A/D conversion circuit; and a predetermined computation is performed with respect to the X-count value, the Y-count value, and the Z-count value by the microcomputer, using calibration coefficients to output a color measurement value as a final output value.
The CRT calibrator is attached to a screen of a CRT monitor via a suction disc in use. In view of this, the CRT calibrator has a small size, and a distance from the CRT screen to the color sensor is relatively short. In addition, the CRT calibrator does not have an optical system such as a lens for reducing a light receiving angle in order to suppress an increase in production cost. As a result, the color sensor has a relatively wide light receiving angle, and light in oblique direction may also be incident onto the color sensor, as well as light in forward direction.
A CRT monitor is constructed in such a manner that an area in the vicinity of a screen surface thereof is illuminated by irradiation of an electron beam, and has a relatively wide view angle. Accordingly, the luminance and the chromaticity of the CRT monitor do not greatly vary depending on an observing angle, and an influence by the wide light receiving angle i.e. the wide view angle of the color sensor is relatively small. Accordingly, the CRT calibrator is capable of precisely measuring a luminance and a chromaticity of the CRT monitor from a low luminance to a high luminance by calibrating a luminance at one predetermined point.
On the other hand, a liquid crystal display device is constructed in such a manner that a backlight device provided on a rear surface of the liquid crystal display device emits light, and liquid crystal elements provided in the vicinity of the screen surface control the light from the backlight device. In the liquid crystal display device having the above arrangement, light in lateral direction which is varied depending on an observing angle may be affected by the light receiving angle of the color sensor. FIGS. 7 and 8 are graphs respectively showing measurement results on a luminance value i.e. an LV value (see FIG. 7) and a chromaticity value i.e. an x-value and a y-value (see FIG. 8) to be obtained by changing the luminance in a state that measurement is performed by the CRT calibrator, and a one-point calibration is performed at 40 cd/m2. The graphs in FIGS. 7 and 8 show error characteristics in a measurement result with respect to a reference sensor having a small light receiving angle (a spectroradiometer CS-1000 of Konica Minolta). In FIGS. 7 and 8, the axis of abscissas indicates a luminance in the unit of cd/m2, and the axis of ordinate indicates an error. As is obvious from FIG. 7, a luminance error is increased, as the luminance is away from 40 cd/m2 as a calibration point. Also, as is obvious from FIG. 8, a chromaticity error is sharply increased, as the luminance becomes smaller than 40 cd/m2. As described above, in the case where a luminance and a chromaticity of a liquid crystal display device are measured by the CRT calibrator, the luminance and the chromaticity may not be precisely measured at a point other than the calibration point.
In view of the above, e.g. Japanese Unexamined Patent Publication No. 2003-294528 (D1) proposes a device for measuring a luminance of a liquid crystal display device. The measuring device has a contact sensor for eliminating an error resulting from a view angle variation of a liquid crystal display device, wherein a measurement result substantially equivalent to a measurement result to be obtained by a sensor of telephoto type having an optical system is obtained by converting an output from the contact sensor, with use of a conversion device such as a lookup table. In view of a point that a conversion value differs depending on the kind of a liquid crystal panel, the above measuring device has multiple conversion devices by the number of the kinds of liquid crystal panels.
The drawback on the light receiving angle may be overcome to some extent by the arrangement of the above related art. However, in the case where a luminance and a chromaticity of a liquid crystal display device are measured by visual observation, or a sensor of telephoto type having a small light receiving angle, combination of the gradation and the brightness of a backlight device may differ depending on the kinds of liquid crystal display devices, although the measurement value indicates an identical luminance value and an identical chromaticity value. For instance, there is a case that the light amount to be transmitted through a liquid crystal display device may be increased, as compared with the liquid crystal display device in a brand new state, because the backlight light amount is lowered resulting from ageing deterioration by long-time use. In this condition, in the case where the luminance and the chromaticity of the liquid crystal display device are measured by the aforementioned color sensor having a wide light receiving angle, a large error in a measurement result may be generated depending on the gradation.
FIG. 9 is a graph showing an error characteristic in a measurement result with respect to a reference sensor (a spectroradiometer CS-1000 of Konica Minolta) having a small light receiving angle, wherein measurement is performed by the CRT calibrator by changing the brightness of a backlight device of a liquid crystal display device. In FIG. 9, the axis of abscissas indicates a luminance in the unit of cd/m2, and the axis of ordinate indicates an error. As is obvious from FIG. 9, even if an identical measurement value is measured at a luminance, an error may be varied depending on combination of the brightness of the backlight device and the gradation. Accordingly, in the technology disclosed in D1, it may be difficult to accurately correct a luminance/chromaticity error, even if a correction coefficient is multiplied to a measurement value.